This application claims the benefit of Korean Application No. P2001-56235 filed on Sep. 12, 2001, which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a liquid crystal display, and more particularly, to a method and apparatus of driving a liquid crystal display. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for improving a picture quality.
2. Discussion of the Related Art
Generally, a liquid crystal display (LCD) controls a light transmittance of each liquid crystal cell in accordance with a video signal, thereby displaying a picture. An active matrix LCD including a switching device for each liquid crystal cell is suitable for displaying a moving picture. The active matrix LCD uses a thin film transistor (TFT) as a switching device.
The LCD has a disadvantage in that it has a slow response time due to inherent characteristics of a liquid crystal, such as a viscosity and an elasticity, etc. Such characteristics can be explained by using the following equations (1) and (2):
xcfx80rxe2x88x9dxcex3d2/xcex94xcex5|Va2xe2x88x92VF2|xe2x80x83xe2x80x83(1) 
where xcfx80r represents a rising time when a voltage is applied to a liquid crystal, Va is an applied voltage, VF represents a Freederick transition voltage at which liquid crystal molecules begin to perform an inclined motion, d is a cell gap of liquid crystal cells, and xcex3 represents a rotational viscosity of the liquid crystal molecules.
xcfx80fxe2x88x9dxcex3d2/K xe2x80x83xe2x80x83(2) 
where xcfx80f represents a falling time at which a liquid crystal is returned into the initial position by an elastic restoring force after a voltage applied to the liquid crystal was turned off, and K is an inherent elastic constant of a liquid crystal.
A twisted nematic (TN) mode liquid crystal has a different response time due to physical characteristics of the liquid crystal and a cell gap, etc. Typically, the TN mode liquid crystal has a rising time of 20 to 80 ms and a falling time of 20 to 30 ms. Since such a liquid crystal has a response time longer than one frame interval (i.e., 16.67 ms in the case of NTSC system) of a moving picture, a voltage charged in the liquid crystal cell is progressed into the next frame prior to arriving at a target voltage. Thus, due to a motion-blurring phenomenon, a moving picture is blurred out on the screen.
Referring to FIG. 1, the conventional LCD cannot express desired color and brightness. Upon implementation of a moving picture, a display brightness BL fails to arrive at a target brightness corresponding to a change of the video data VD from one level to another level due to its slow response time. Accordingly, a motion-blurring phenomenon appears from the moving picture and a display quality is deteriorated in the LCD due to a reduction in a contrast ratio.
In order to overcome such a slow response time of the LCD, U.S. Pat. No. 5,495,265 and PCT International Publication No. WO99/05567 have suggested to modulate data in accordance with a difference in the data using a look-up table (hereinafter referred to as high-speed driving scheme). This high-speed driving scheme allows data to be modulated by a principle as shown in FIG. 2.
Referring to FIG. 2, a conventional high-speed driving scheme modulates input data VD and applies the modulated data MVD to the liquid crystal cell, thereby obtaining a desired brightness MBL. In the high-speed driving scheme, |Va2xe2x88x92VF2| is increased from the above equation (1) on the basis of a difference of the data so that a desired brightness can be obtained in response to a brightness value of the input data within one frame interval, thereby rapidly reducing a response time of the liquid crystal. Accordingly, the LCD employing such a high-speed driving scheme compensates for a slow response time of the liquid crystal by modulating a data value in order to alleviate a motion-blurring phenomenon in a moving picture, thereby displaying a picture at desired color and brightness.
In other words, the high-speed driving scheme compares most significant bits MSB of the previous frame Fnxe2x88x921 with those of the current frame Fn. If there is a change in the most significant bits, the corresponding modulated data Mdata are selected from the look-up table to modulate the data as shown in FIG. 3. The high-speed driving scheme modulates only several most significant bits to reduce a memory size upon implementation of hardware equipment. A high-speed driving apparatus implemented in this manner is as shown in FIG. 4.
Referring to FIG. 4, a conventional high-speed driving apparatus includes a frame memory 43 connected to a most significant bit bus line 42 and a look-up table 44 commonly connected to the most significant bit bus line 32 and an output terminal of the frame memory 43.
The frame memory 43 stores most significant bit data MSB during one frame interval and supplies the stored data to the look-up table 44. Herein, the most significant bit data MSB may be the most significant 4 bits of the 8-bit source data RGB.
The look-up table 44 compares most significant bits MSB of a current frame Fn inputted from the most significant bit line 42 with those of the previous frame Fnxe2x88x921 inputted from the frame memory 43 as shown in Table 1 or Table 2, and selects the corresponding modulated data Mdata. The modulated data Mdata are added to least significant bits LSB from a least significant bit bus line 41.
In the above tables, a left column is for a data voltage VDnxe2x88x921 of the previous frame Fnxe2x88x921 while an uppermost row is for a data voltage VDn of the current frame Fn. Table 1 is a look-up table information in which the most significant bits (i.e., 20, 21, 22 and 23) are expressed by the decimal number format. Table 2 is a look-up table information in which weighting values (i.e., 24 25, 26 and 27) of the most significant 4 bits are applied to 8-bit data.
However, the conventional high-speed driving scheme is problematic. Since it has been studied on the assumption that a driving frequency of the data is fixed like a television, the conventional scheme is difficult to be applied in a frequency-variable display device which receives different driving frequencies such as a computer monitor. More specifically, in the conventional high-speed driving scheme, a voltage level of the modulated data Mdata is fixed to a specific frequency (e.g., 60 Hz) and a response time (i.e., 16.7 ms) of the liquid crystal is fixed in accordance with the specific frequency. On the other hand, a computer monitor is manufactured so that its driving frequency can be changed in the range of 50 to 80 Hz. Therefore, in order to apply the conventional high-speed driving scheme to such a computer monitor, the modulated data Mdata established in the conventional high-speed driving scheme should be modified depending on a driving frequency. This is because a voltage charged in a liquid crystal should be changed depending on a driving frequency to vary a response time of the liquid crystal. As a result, when the modulated data Mdata established based on only a specific driving frequency is applied to a monitor displaying a picture at a driving frequency different from the specific frequency, a picture is more deteriorated.
Accordingly, the present invention is directed to a method and apparatus for driving a liquid crystal display that substantially obviates one or more of problems due to limitations and disadvantages of the related art.
Another object of the present invention is to provide a method and apparatus for driving a liquid crystal display that improves a picture quality.
Additional features and advantages of the invention will be set forth in the description which follows and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, a method of driving a liquid crystal display includes determining modulated data in accordance with one of a driving frequency and a driving frequency band having a desired frequency range, separately registering the modulated data in a plurality of look-up tables separated for any one of the driving frequency and each driving frequency band having the desired frequency range, detecting the driving frequency, and selecting one of the modulated data outputted from the plurality of look-up tables in accordance with the detected driving frequency to modulate source data.
The method further includes dividing the source data into most significant bits and least significant bits, and delaying the most significant bits.
In the method, the plurality of look-up tables compare the delayed most significant bits and non-delayed most significant bits to select one of a plurality of modulated data registered in advance in accordance with the compared result.
In another aspect of the present invention, a driving apparatus for a liquid crystal display includes a mode detector detecting a driving frequency of source data, a plurality of look-up tables having registered modulated data determined for one of a driving frequency and a driving frequency band having a desired frequency range to modulate the source data, and a switch selecting one of the modulated data from the look-up tables in accordance with the detected driving frequency and outputting the selected modulated data.
The driving apparatus further includes a frame memory delaying most significant bits of the source data for one frame period and outputting the delayed most significant bits to the plurality of look-up tables.
In the driving apparatus, each of the plurality of look-up tables compares the delayed most significant bits with non-delayed most significant bits to select modulated data corresponding to the source data.
The driving apparatus further includes a data driver applying data outputted from the switch to a liquid crystal display panel, a gate driver applying a scanning signal to the liquid crystal display panel, and a timing controller applying the source data to the plurality of look-up tables and the mode detector and controlling the data driver and the gate driver.
In a further aspect of the present invention, a liquid crystal display includes a liquid crystal display panel displaying images, a mode detector detecting a driving frequency of source data, a frame memory delaying most significant bits of the source data for one frame period and outputting the delayed most significant bits of the source data, a plurality of look-up tables having registered modulated data determined for one of the driving frequency and a driving frequency band having a desired frequency range, comparing the delayed most significant bits with non-delayed significant bits of the source data, and outputting one of the registered modulated data from each look-up table based on the compared result, and a switch selecting the one of the registered modulated data in accordance with the detected driving frequency and outputting the modulated data to the liquid crystal display panel.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.